The invention relates to a steel alloy for a ferritic steel with excellent creep strength and oxidation resistance at elevated usage temperatures.
More particularly, the invention relates to seamless or welded pipes from the steel alloy, which are used, for example, as heat exchanger pipes in heaters or power plant boilers in temperature ranges of above 620° C. to about 750° C.
High-temperature materials with high creep strength and corrosion resistance for, for example, application in power plants are based generally either on ferritic, ferritic/martensitic or austenitic iron-based alloys or on nickel-based alloys. The specific requirements in the lower temperature stages of the heat exchanger pipes relate in particular to a small thermal expansion.
Austenitic materials cannot be used because their thermal expansion is too high in the aforedescribed temperature range. The ferritic/martensitic materials available to date can also not be employed in the boiler at the enhanced temperatures, because their creep strength and heat resistance combined with adequate corrosion resistance are no longer sufficient.
Nickel-based alloys with nickel content of more than 50 wt.-% represent an adequate combination of corrosion resistance and heat resistance properties. These steels are therefore extremely expensive and processing into seamless pipes is also quite problematic.
Pipes made of austenitic steels with low requirements for thermal expansion have been used to date for components in power plant boilers. The high alloying costs (Ni to 30%), the inferior machinability and the inferior thermal conductance are here disadvantageous.
Chromium-rich ferritic steel is significantly less expensive than austenitic stainless steel, while also having a higher thermal conductivity coefficient and a lower thermal expansion coefficient. In addition, chromium-rich ferritic steel also has a high oxidation resistance which is advantageous when used with hot steam, for example in heaters or boilers.
However, when oxide layers are produced in form of a coating (scale or scale layer), then these oxide layers can detach when the boiler temperature and/or the boiler pressure change, and get stuck in and plug up the steel pipes.
In addition to the required creep strength and heat resistance, suppressing oxidation from steam is therefore one of the problems that foremost require a solution.
For improving the efficiency of the energy generation in power plants, there is increasing a requirement to increase the steam temperature to above 620° C. and to also increase the steam pressure in the boiler.
The market forces hence require ferritic iron-based alloys for pipes and/or pipelines which exhibit the required creep strength and corrosion properties also at higher usage temperatures above 620° C. For example, creep strengths of 105 hours at this temperature exposure for a load of 100 MPa should be attained without cracking.
Steels available for a usage temperature up to about 620° C. and 650° C., respectively, are ferritic/martensitic steels with Cr-contents of, for example, 8 to 15%.
Corresponding steels are disclosed, for example, in the documents DE 199 41 411 A1, DE 692 04 123 T2, US 2006/0060270 A1, DE 601 10 861 T2 and DE 696 08 744 T2. The alloying concepts disclosed therein involve mostly expensive alloying additives or are also not suitable for use in temperature ranges above 620° C.
Concepts based on incoherent MX- or M2X-precipitates for increasing the creep strength (DE 199 41 411 A1, DE 601 10 861 T2, US 2006/0060270 A1) have several disadvantages.
The aforementioned precipitation phases cannot be produced in sufficient volume fractions, because an increase of the contents of the metallic (e.g., Ti, Nb or V) as well as the non-metallic components (C or N) does not only increase the phase fraction, but also increases the solution temperature of the phase. The creation temperature of the precipitates is then above a realistic heat treatment temperature and partially also above the solidus temperature of the alloy.
Because the temperature at which precipitates are produced is directly related to their size, one either obtains a relatively small volume fraction of effective reinforcing particles (<1%) or a high volume fraction of coarse particles (>1 μm), which have no effect on the creep strength. The MX- and M2X-particles precipitate preferably in the interior of the grain. It can be expected that the influence from grain boundary creep relative to the creep caused by dislocations increases at usage temperatures of >630° C.
A depletion of reinforcement phases at grain boundaries therefore deserves a particularly critical evaluation.
Moreover, the incoherent precipitates have a greater tendency to become coarser than coherent precipitates because, on one hand, the boundary surface energy as a driving force for minimizing boundary surfaces is greater than for coherent particles and, on the other hand, easily diffusing elements, such as C and N, are a component of these particles.
Other conventional alloying concepts that use intermetallic phases for increasing creep strength of ferritic or martensitic steels (DE 698 08 744 T2) are based on expensive alloying materials.
For adjusting a sufficiently high volume fraction of intermetallic phases with the structure L10 or L12, the extremely expensive alloying elements Pt and Pd, which have to date only been available in small quantities, with fractions about 1 wt.-% are required.
The alloy described in WO 03/029505 is an improvement over the FeCrAl-alloy known under the name Kanthal, which is used, for example, for heating elements operating at temperatures above 1000° C. These alloys have a high chromium and aluminum content for efficiently converting electric energy into heat.
The combination of high chromium and aluminum contents results in alloys that with chromium contents above 16% and aluminum contents above 4% are fully ferritic even at temperatures above 750° C. The steels are not suitable for use in power plant applications; moreover, chromium contents above 16% worsen the deformability at typical processing temperatures when rolling seamless pipes (900-1200° C.). This diminished deformation characteristic can result in crack formation during rolling. As a result, such alloys are not suitable for the production of pipes or sheet metal.
U.S. Pat. No. 6,322,936 B1 describes exclusively intermetallic alloys produced by powder metallurgy for the production of sheet metal based on the system Fe—Al and includes the intermetallic phases Fe3Al, Fe2Al5, FeAl3, FeAl, FeAlC, Fe3AlC, and combinations of these phases. A disordered phase, for example ferrite, is not included. The described FeAl—B2 phase is in these documents used only as a matrix. The powder-metallurgical production of such intermetallic alloy is not suitable for the large-scale production of pipes and sheet metal.
It is an object of the invention to provide a cost-effective steel alloy for a steel which is ferritic at the usage temperature and which reliably satisfies the aforedescribed requirements with respect to the creep strength and oxidation resistance also at usage temperatures of up to about 750° C.
It is another object to provide workpieces produced with this steel alloy, for example hot-rolled seamless or welded pipes, sheet metal, cast workpieces or tool steels.